1. Get in a plank position - hands directly under your shoulders, and not ahead of them, like a lot of women do.
2. Imagine that you have a board on your back side which runs from the top of your head to just above your feet, and it's strapped on with a big, elastic band is about 5 inches wide. You are now forced to keep your back straight, and your belly can't sag. Don't let your legs sag either. Your core is strong, and so is your back.
3. Keep your weight over your hands. Memorize how that feels - this is your starting position for a push-up. In the beginning, you can keep your feet wider apart.
4. Now try to do an actual push-up: while keeping your knees straight and your butt not a mountain or valley, bend your arms until you are parallel, then push back up. If you feel lots of tension in your tricep/bicep area, then that means that you're doing it right.
5. A good way to work into straight-leg push-ups is to do decline push-ups using a stability ball. Start with the ball under your upper thighs, then move it down to under your shins. Then do regular push-ups, and skip the ones on your knees.
6. Stop telling yourself you can't do them - imagine you're going into the army and they've raised the enlistment age to 50. You have to learn how, no excuses will be accepted. They don't care if you're tall or short - you just plain have to learn how to do them. A drill sargeant will not listen to you lament about your height or your advanced age!
"It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt." Mark Twain ;-)